It's justified in this case though, as that girl is more mentally ill than in love. She used to be completely alone until he brought her into his group of friends as a kid, and since then she's been something in between that and her waking him up by offering herself to him (even if he locked the door when he went to bed).

I think they know whats going on internally, but they just try to be respectable and all that. Its what makes the protagonist seem trust-able cus he cares more about people then just duh pussy and stuff. Also ********** is right, if they didn't act that way it would basically be the plot of a hentai.